LEAF GAS-EXCHANGE OF DROUGHTED AND IRRIGATED BANANA CV WILLIAMS (MUSASPP.) GROWING IN HOT, ARID CONDITIONS

Leaf gas exchange (LGE) of banana plants cv. Williams growing in an ar id tropical environment was measured on irrigated and droughted plants through an 8 d drying cycle and after re-irrigation. The relations be tween net photosynthesis (P-n), stomatal conductance (g(s)), transpira tion (E-t) and internal CO2 concentration (C-i) and climatic factors ( photosynthetic photon flux density (PPFD), leaf temperature (T-1) and leaf-to-air vapour pressure difference (Delta e) and plant and soil wa ter status were explored. The highest P-n, of 23 mu mol CO2 m(-2) s(-1 ) and g(s) of 600 mmol m(-2) s(-1), occurred in early to mid morning b ut the highest E-t, of 10 mmol H2O m(-2) s(-1), occurred in the early afternoon. Water deficit significantly (P less than or equal to 0.05) reduced LGE when soil water potential (Psi(S)) at 30 cm soil depth was less than -30 kPa. The Psi(S) of irrigated plants was maintained abov e -20 kPa throughout the experiment. Water use efficiency was not sign ificantly (P less than or equal to 0.05) different between treatments. Nor was the relation between the difference between leaf and air temp erature (T-d) and Delta e. A unique relation existed between P-n and g (s) that was common to both irrigated and droughted plants. Soil droug ht, and in irrigated plants diurnal variation in Delta e and T-1, redu ced P-n and g(s). C-i was positively correlated with both P-n and g(s) except when the plants had experienced more than 6 d of drought (Psi( S) lower than -50 kPa) when the relations became non-significant (P le ss than or equal to 0.05). After re-irrigation of the droughted plants these relations returned to being positive and statistically signific ant (P less than or equal to 0.05). Measurements of leaf water potenti al using either volumetric or pressure techniques rarely showed differ ences between irrigated and droughted plants. This was despite a large reduction in P-n, E-t and g(s). Mature banana leaves folded their lam inae to a greater extent, and for longer periods during the day, in dr oughted plants but we suggest leaf folding may be too variable over th e diurnal period to be used as a plant-based indicator of water defici t. The youngest leaf emerged up to 40% faster during the night than th e day suggesting that day temperatures were above the optimum for leaf elongation. Drought significantly (P less than or equal to 0.05) redu ced elongation of emerging leaves only when Psi(S) was less than -65 k Pa. In irrigated plants P-n was reduced whenever g(s) declined due to diurnal changes in the environment. Psi(S) less than -30 kPa further r educed LGE in droughted plants but few associations with plant water s tatus (either pressure or volume based measurements of leaf water stat us or plant morphology) could be linked with decreased LGE. We conclud e that banana plants are well able to maintain their internal water st atus during drought and are sensitive to soil drying. Water status is maintained by reducing radiation load (leaf folding) and closing stoma ta. This is likely to bear strongly on the productivity of this crop i n hot, arid environments.